Conventionally, an inkjet recording apparatus records an image on a recording media by repeating a main scan and a sub-scan. While the main scan, a head ejects ink to the recording medium and is reciprocated in a main scanning direction. The sub-scan conveys the recording medium in a sub-scanning direction. In this type of inkjet recording apparatus, image quality of a higher resolution than the pitch of nozzles arranged in the sub-scanning direction is sometimes required. Further, recording is carried out so as to fill a dot space formed by one main scan by the next main scan onward, and a recording method called interlace is known as one of the recording methods.
This interlace will be described as shown in FIG. 9A. As shown at the left of FIG. 9A, description will be given of a method for performing recording so as to satisfy a required resolution of 600 dpi by use of 93 nozzles in a head 100 having nozzles P pierced at a pitch of 1/150 inches in a sub-scanning direction B.
In this case, as illustrated at the right of FIG. 9A, a recording medium is conveyed by 93/600 inches in the sub-scanning direction B after one main scan (first pass). Then, after the next main scan (second pass) is carried out, the recording medium is again conveyed by 93/600 inches in the sub-scanning direction B. Recording continues while repeating such a main scan and sub scan. The diagram at the right of FIG. 9A illustrates a condition of the head 100 relatively moving with respect to the recording medium conveyed in the sub-scanning direction B, and the head 100 is illustrated in a manner displaced rightward for each main scan. Besides, the head 100 exists on an identical line at a start position of each main scan in actuality.
FIG. 10 is a diagram showing the condition illustrated at the right of the FIG. 9A in a further enlarged manner. The leftmost column of FIG. 10 indicates 1/600 inches per square. In addition, the column is omitted halfway in places for illustration. The adjacent right-hand column of the leftmost column shows nozzle numbers (blackened parts) to eject ink in the first pass. The adjacent right-hand column thereof shows nozzle numbers (blackened parts) to eject ink in the second pass after conveying the recording medium in the sub-scanning direction B by 93/600 inches after the first pass. The adjacent right-hand column thereof shows nozzle numbers (blackened parts) to eject ink in the third pass after conveying the recording medium in the sub-scanning direction B by 93/600 inches after the second pass. The adjacent right-hand column thereof shows nozzle numbers (blackened parts) to eject ink in the fourth pass after conveying the recording medium in the sub-scanning direction B by 93/600 inches after the third pass.
According to the above, a state that ink is ejected from the 70th nozzle and the 71st nozzle in the first pass, ink is ejected from the 47th nozzle in the second pass, ink is ejected from the 24th nozzle in the third pass, and ink is ejected from the 1st nozzle in the fourth pass is detected, when attention is focused on the position of 276 to 280 (K part) in the leftmost column of FIG. 10.
The ink that is ejected from the 47th nozzle in the second pass is ejected 1/600 inches upstream in the sub-scanning direction B of the ink that has been ejected from the 70th nozzle in the first pass and has been ejected on the recording medium. The ink that is ejected from the 24th nozzle in the third pass is ejected 1/600 inches upstream in the sub-scanning direction B of the ink that has been ejected from the 47th nozzle in the second pass and has been ejected on the recording medium. The ink that is ejected from the 1st nozzle in the fourth pass is ejected 1/600 inches upstream in the sub-scanning direction of the ink that has been ejected from the 24th nozzle in the third pass and has been ejected on the recording medium. In other words, the ink that is ejected from the 1st nozzle in the fourth pass is ejected 1/600 inches downstream in the sub-scanning direction B of the ink that has been ejected from the 71st nozzle in the first pass and has been ejected on the recording medium.
FIG. 11A is a diagram showing a condition of the ink ejected from the K part of FIG. 10 having been ejected on a recording medium. Ejection drops D1 and D5 are formed at a pitch of 1/150 inches in the first pass. An ejection drop D2 is formed upstream in the sub-scanning direction B at a pitch of 1/600 inches with respect to the ejection drop D1 in the second pass. An ejection drop D3 is formed upstream in the sub-scanning direction B at a pitch of 1/600  inches with respect to the ejection drop D2 in the third pass. An ejection drop D4 is formed at a pitch of 1/600 inches with respect to the ejection drop D3 in the second pass. In this manner, recording can be performed so as to satisfy the required resolution 600 dpi by use of the 93 nozzles P in the head having nozzles P pierced at a pitch of 150 dpi.
However, in a case of recording by the interlace technique described above, the following problems have existed. FIG. 11B is a sectional diagram along an A-A section line of FIG. 11A. In the case of recording by interlacing, the ejection drop D2 to be formed in the second path is formed so as to be contiguous upstream in the sub-scanning direction B with respect to the ejection drop D1 formed in the first pass. Therefore, when the ejection drop D2 is formed before the ejection drop D1 has dried, the ejection drop D2 is drawn toward the ejection drop D1 (arrow Z direction) due to the effect of surface tension and the like of the ejection drop D1 (see the dotted line of the ejection drop (D2)). Accordingly, overlap between the ejection drop D1 and ejection drop (D2) increases. Further, overlap between the ejection drop (D2) and ejection drop D3 decreases. Therefore, a streaky unevenness of ink (banding) occurs and recording quality is degraded. With respect to this type of problem, JP-A-07-47677 discloses a method for performing recording so that recording dots in the sub-scanning direction are not contiguous to each other.